TCAS2 (Traffic alert and Collision Avoidance System 2) is known as a system for avoiding midair collision of aircrafts. The aircraft installed with TCAS2 inquires to other aircrafts around the own aircraft and responds to the inquiry from the other aircraft. Based on the inquiry and response, TCAS2 judges whether or not other aircrafts having a risk of the midair collision exist around the own aircraft, and warns a pilot of the existence of the other aircrafts having the risk of the midair collision. The installation of TCAS2 is obligated in a turbine aircraft on which 19 or more passengers board.
However, because the installation of TCAS2 is not obligated to all aircraft, there is a problem of avoidance of midair collision with another aircraft without the installation of TCAS2. In a manned aircraft that a pilot is on board, it is possible to avoid the midair collision with the aircraft without the installation of TCAS2 through visual surveillance of the neighborhood by the pilot. Because an unmanned aircraft is controlled through a remote control by a pilot who is not on board, it is required to install a surveillance system in the unmanned aircraft in place of the visual surveillance.
U.S. Pat. No. 5,581,250 discloses a technique to avoid midair collision by using a TV (television) camera which monitors the front direction of the unmanned aircraft. According to this U.S. Pat. No. 5,581,250, two TV cameras are installed in both wings of the unmanned aircraft for surveillance of the front direction so as to calculate a distance to an obstacle in front of the unmanned aircraft based on images acquired from these TV cameras. However, it would be difficult to guarantee a necessary range accuracy due to the influence of twist of wings.
“DSA Radar-Development Report” (AUVSI 2005 Jun. 26 to 30, 2005) discloses a technique that when another aircraft exists in front of the unmanned aircraft, a distance to the other aircraft and an azimuth thereof are detected by using a millimeter-wave radar which is installed in the neighborhood of the nose of the unmanned aircraft, so as to avoid the midair collision. However, it would be difficult to apply the millimeter-wave radar to a small unmanned aircraft because the millimeter-wave radar is heavy and large in a consumed power amount.